In optical recording, increasingly the trend is towards miniaturisation of both the optical record carriers and the devices used to scan (e.g. write to and/or read from) the carriers. Examples of optical record carriers include CDs (compact discs) and DVDs (digital versatile discs).
In order for the optical record carriers to be made smaller, without a decrease in information storage capacity, the information density on the carrier must be increased. Such an increase in information density must be accompanied by a smaller radiation spot for scanning the information. Such a smaller spot can be realised by increasing the numerical aperture (NA) of the objective system used for focusing a radiation beam in the scanning device on the record carrier. Consequently, it is desirable to have a lens with a high numerical aperture (e.g. NA=0.85).
Conventional high NA objectives consist of two elements in order to ease the manufacturing tolerances, at the expense of introducing an extra assembly step to align the elements making up the objective lens.
The Japanese article “Single Objective Lens Having Numerical Aperture 0.85 for a High Density Optical Disk System” by M Itonga, F Ito, K Matsuzaki, S Chaen, K Oishi, T Ueno and A Nishizawa, Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 1798–1803 Part 1, No. 3B March 2002, describes a single objective lens, having two aspherical surfaces, with a relatively high NA of 0.85. The lens is made of glass. The lens diameter is 4.5 mm, and the lens has an aperture diameter of 3.886 mm. This single element lens does not require the alignment assembly step needed by the two-element objective. Because of the high value of NA, the objective lens becomes more susceptible to variations in the manufacturing process i.e. manufacturing tolerances. Therefore, for these high NA objectives the manufacturing tolerances play an even more important role in the designing process than was the case for objectives having a lower numerical aperture.
In order for scanning devices to decrease in size, it is desirable that the components within the scanning devices (such as the objective lens) are made as small as possible.
However, it is not possible to simply scale down large lens designs to produce smaller lenses, as the lens design is dependent upon the properties of the optical recording medium. For instance, the lens design is dependent upon the properties of the transparent layer that typically covers the information layer on an optical record carrier, and which the scanning radiation beam must traverse. In the scaling down process the thickness of the cover layer of the disc remains unaffected (the same record carrier is likely to be used for both the normal sized objective and the small sized objective). Hence, the design of a small sized objective suitable for scanning the optical record medium will be substantially different from the design of a normal sized objective.
It will also be appreciated that as lenses are made smaller, the high NA objective lenses remain susceptible to variations in the manufacturing process i.e. manufacturing tolerances.
FIG. 1A shows an example of an objective lens 18, having two aspherical surfaces 181, 182 and of thickness t (the lens thickness along the optical axis 19). Subsequent FIGS. 1B, 1C and 1D respectively illustrate how the lens shape will vary due to variations in thickness, decentre and tilt of the two aspherical surfaces (in each instance, the original position of the surface 181 is illustrated by a dotted line). In these Figures, it is assumed that only the surface 181 has been affected by the variations in the manufacturing process. However, it will be appreciated that in actual fact either or both of the surfaces can be affected, and that either surface could be affected by two or more of these deviations simultaneously.
FIG. 1B illustrates the thickness of the lens being greater than the desired thickness t, due to the spacing in between the aspherical surfaces being larger than desired. However, it will be appreciated that the two aspherical surfaces could in fact be spaced closer together than desired as well.
FIG. 1C illustrates decentre i.e. in this example, how surface 181 has been formed shifted in a direction perpendicular to the ideal position relative to the desired optical axis 19.
FIG. 1D illustrates how surface 181 is tilted i.e. rotated in relation to the desired rotationally symmetric position along the principal axis.
It is an aim of embodiments of the present invention to provide an objective lens formed from a single material capable of withstanding reasonable manufacturing tolerances.
In optical scanning devices, radiation beams may enter the objective lens obliquely, due to inaccurate alignment of the objective lens within the scanning device, variations in the position of the recording carrier relative to the scanning device, or due to radiation beams being utilised that do not travel along the optical axis. For instance, such off-axis beams are typically used to provide information on positioning of the scanning radiation spot on the record carrier.
Such oblique beam entrance results in wave-front aberrations. Typically an allowance in the root mean square of the optical path difference (OPDrms) of approximately 0.07 λ (where λ is the wave length of the relevant radiation beam), in total is allowed for wave-front aberrations of the scanning beam for the total optical scanning device, such that the system is diffraction limited. It can be convenient to express the OPDrms in mλ (where 0.001λ=1 mλ). The field of the lens system is the area within which oblique beams generate an OPDrms of less than 15 mλ. The field of view of the lens system is twice the field.
It is an aim of the embodiments of the present invention provide a small sized high NA objective lens formed from a single material that is tolerant to oblique beam entrance to the lens and tolerant for manufacturing errors.